1. Field of the Invention
The present invention relates to a thread trimming device for a lockstitch bar tacking sewing machine.
2. Description and Problems of the Related Art
A lockstitch bar tacking sewing machine performs a cycle of sewing operation to form a given number of stitches for bar tacking sewing, button sewing, etc., and there are known some sewing machines of this kind equipped with thread trimming devices for trimming the needle and bobbin threads upon completion of a cycle of sewing operation which corresponds to one revolution of a feed cam. Thread trimming devices of conventional sewing machines of this kind are designed to perform thread trimming by driving a movable knife provided under a throat plate to turn by way of a link equipped with a roller which follows the variation of cam surface of a thread trimming cam mounted on an arm shaft only at the last stitch during a cycle of sewing operation corresponding to a given number of stitches due to the variation of the cam surface on the side of a feed cam. The roller is connected to the movable knife by way of a mechanical link mechanism.
Whereas excepting the time for the thread trimming, i.e., excepting the time for forming the last stitch during a cycle of sewing operation, the cam surface of side surface of the feed cam coercively holds the link equipped with the roller for preventing the roller from touching the cam surface of the thread trimming cam so as to prevent the thread trimming.
Such a contact between the roller and the feed cam not only increases the torque of the arm shaft but also generate abrasion on the cam surface of the side surface of the feed cam. Moreover, since the mechanism for preventing the thread trimming during a cycle of sewing operation excepting the time of forming the last stitch is composed of the mechanical link mechanism alone, there is a technical problem that it is complicated in construction and requires much labor in adjusting operation and part exchanging operation.